Hyponatremia is among the most comon conditions found in clinical medicine. The main goal of this project is to study the effect on the brain of the adaptation to acute and chronic hyponatremia, and the brain's response to correction of hyponatremia. We have shown in a rat model that rapid increases in plasma sodium concentration or plasma osmolality can disrupt the blood brain barrier (BBB); that this disruption occurs at a lower plasma osmolality in chronic hyponatremic rats than in normonatremic controls; that a rapid increase in plasma sodium and plasma osmolality causes a marked global increase in cerebral perfusion in both hyponatremic and normonatremic rats; and that disruption of BBB appears to be related to the subsequent development of brain demylenation. Given these findings, we have begun a series of experiments to study how alterations in BBB permeability and the changes in cerebral perfusion which follow rapid changes in plasma osmolality relate to the subsequent development of neurologic symptoms and brain demylenation which often follow rapid correction of hyponatremia. Specifically, we hope to identify the mechanisms responsible for the osmolar induced increase in cerebral perfusion, and to determine the mediators responsible for the changes in cerebral perfusion during correction of hyponatremia.